So I want to use the non invasive method of a coil attatched as close as possible to the earpeice of a house phone so that I can record or put on speaker what is being said on the other end. what sort of amplification will I need and what type of coil for example roughly how many windings of what size wire ?
or if a direct connection to the earpeice is a better solution what should I do I don't fancy putting 40 volts into and op amp that will only take 20 for example. If I remember correctly there are dc 40 V when the line is inactive that go to 8 volts when it is active so would a blocking condenser be sufficient ?
oops not much of a chance of laying hands on one of them, what if I an op amp ,the op amp has a high input impedence so shouldn't disturb the phone, what voltages are involved ?
will this diaghram give good results in amplifiing ? it should have (virtual) infinit input imedence and be able to acheive very high amplification as necesary
Your 1st opamp doesn't have a DC reference voltage at its input so it won't work.
The phone line is balanced to cancel interference and mains hum. Your circuit is unbalanced and if it worked then it would just amplify the interference and hum.
You need a transformer to balance and isolate the phone line.
That's exactly how a pickup coil works, the earphone voice coil is the primary and the pickup coil is the secondary, you shouldn't need any amplification for a tape recorder but if it's needed a then a normal pre-amp will probably do.
the off hook voltage is -48vdc (red to green phone wires)
and the ring voltage is +90vac
good to know if youre gonna connect anything more than a pickup coil
ok audioguru what do I have to do to that circuit assuming I will connect it to a coil, am thinking of an old earphone. how do I balance it I thought those were standard non inverting and inverting op amp configurations, how do I balance it or is it better to use a dual power supply
A coil is the secondary of a transformer where the telephone's earphone is the primary, so your amplifier doesn't need to be balanced. The coil provides perfect balancing.
I thought those were standard non inverting and inverting op amp configurations, how do I balance it or is it better to use a dual power supply
Your 1st opamp didn't have a DC reference voltage for its input.
Opamps don't need a dual power supply. They just need their input DC voltage about half-way between ground and the positive supply when using a single supply circuit. Look at these schematics:
thats vvery interesting thanks,
but now I am confused I've always seen the non inverting config like I drew it up and you say that if a coil is the input that is ok why ?
would I need your version if it was getting it's input for example from another op amp ?
do I need a coupling capacitor between the stages ?
A coil is completely separate from the balanced telephone because its coupling is a magnetic field so the coil doesn't unbalance the telephone which would cause hum.
do I need a coupling capacitor between the stages ?